Graphitization of polymers offers an efficient way to synthesize nanocrystalline graphene on different substrates with tunable shape, thickness and properties. We have extended this approach to study the catalyst free thermal formation of free-standing nanocrystalline graphene films and followed the graphitization using in situ TEM techniques, combining atomic and high resolution imaging, diffraction and electron energy loss spectroscopy. In situ graphitization allowed understanding the chemical and structural evolution of the nanocrystalline graphene during the pyrolysis process. The in situ studies showed that the graphitization process is highly dynamic with a number of intermediate reactions leading to the formation of different carbon nanostructures. Both mobile and stationary (pinned) structures with varying size and shape were observed.

In situ high resolution transmission electron microscopy at high temperatures provides an understanding of the stability/mobility of individual graphitic subunits and their interactions to understand the fundamental processes controlling graphene growth. Small graphitic subunits are highly mobile on the graphitic substrate and move/attach to the edges of a graphene flake, extending the flake. Studying these structures at high temperatures provided insights in to the different mechanisms responsible for the growth of graphitic domains at high temperature. The study revealed that the growth of the domains is mainly by the migration and merging of the graphitic subunits. In addition to this, strong structural and size fluctuations of individual graphitic subunits at high temperatures are observed. Graphene nano flakes are highly unstable and tend to loose atoms or groups of atoms to adjacent larger domains probably indicating also an Ostwald type of ripening in these 2D materials. Beam off heating experiments were carried out to understand the effect of beam on the observed dynamics, to separate out the beam induced transformations and the inherent temperature driven mechanisms.